The chemical induction of mammary carcinoma in rats has been studied extensively as a model for human breast carcinoma. A great deal of information has been gained regarding the hormonal requirements for tumor induction, growth and metabolism. Because of the complex inter- relationships between various hormones, a simpler in vitro organ culture system has been sought to more directly study the relation between hormone, carcinogen, and target organ during the initiation of tumorigenesis as well as the factors regulating tumor growth and metabolism. We hope that such an architecturally intact system would be suitable to study: 1) the possible mitogenic effect of estrogens; 2) the hormonal conditions responsible for tumor initiation and progression; 3) the hormonal role in the early process of carcinogenesis as measured with the partial inhibition of RNA synthesis; 4) the behavior of hyperplastic nodules induced in vivo and in vitro, and the factors governing their progression into tumor; and 5) the hormonal requirement for maintenance of activity of selected enzymes of mammary tumors.